ON THE STRUCTURE OF THE BRAIN OF THE SESSILEEYED CRUSTACEA. 



JthUD AT WAfiHINGION, APRIL 14, 1SS4. 



Ry A. S. Packaiu) 



The following descriptions and notes liave grown ont of an attempt to compare the nervous 

 system, particularly the brain and other ganglia of the head, of the eyeless sjiecies of cave-inhabiting 

 Arthropods with their out of-door allies. We have begun with the structure and morphology of 

 the brain of Asellus communis Say as a standard of comparison with that of the blind Asellid, 

 Cecidotwa sti/ffia Pack., which is so common in the brooks of Mammoth and other caves and in the 

 wells of Southern Indiana and Illinois. Studies of this nature are, it seems to us, well calculated 

 to throw light on the origin of the cave forms, and to show what great moditications have been 

 produced in these organisms by a radical change in tlieir surroundings ; consisting, as it does, mainly 

 in the absence of light, and perhnps of the usual food, or at least the usual amount of food. 



It is plain enough that tlie species of Cecidota^a are simply eyeless, slender, depauperated 

 Aselli, which have originated from some one of our out-ofdoor species within a comparatively 

 recent time, at least since the river-terrace epoch of the Quaternary Period. The facts bearing 

 upon the general relations of the blind to the eyed Asellidie, and a discussion of the change in 

 form of the body and its appendages, and of the causes of the transformation of the species and 

 genus, are reserved for another occasion. 



My iiresent purpose is simply to describe and depict the brain and other nerve-centers of the 

 head of Asellus communis Say aiul Cecidotwa styyia Pack. 



I. The beain of Asellus communis. 



The nervous system of the European Asellus aquaticus Linn, has been referred to by Leydig 

 and also by Sars, who published a figure of the nervous system as a whole. Leydig's " Vom Ban 

 des thierischen Kiirpers''^ gives a careful and comprehensive general account of the nervous system of 

 Arthropods, the most complete and authoritative, up to 18C4, we possess, suppleaiented as it is by 

 his excellent Tafeln von vergleichenden Anafomie, published in the same year (1864). According to 

 Leydig, in the Isopoda (Oniscus, Porcellio) the optic lobes are very large and overlie the cerebral 

 lobes. 



In Asellus aquaticus the abundant fat body around the ventral cord belongs to the blood sinus 

 which envelops the nervous cord. Of this form Leydig has little to say, remarking that he did not 

 examine the entire ventral coi'd, but only sections, which agree in appearance with those of the 

 land wood-lice. 



Sar's figure of the brain of Asellus aquaticus is drawn on a small scale, is rather indifferent, and 

 does not show more than the cerebral lobes and optic nerves. He evidently did not perceive the 

 other ganglia. 



Leydig's valuable figures of the brain of Oniscus murarius show that he did not study the 

 nervous centers of the head by means of lougitudinal sections, and that he simply dissected the 

 brain from above, a dorsal view showing the large optic lobes to be mostly above and in front of 

 the smaller cerebral lobes, while the ganglion, e, in his figure S (Taf. VI), which he denominates 

 nebenlappen^ is probably one of the autenual ganglia. The other ganglia of the head he does not 

 rei^resent, nor sx^eak of iu his Vergleichende Anatomic. 



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